Co-op progress issue in campaign

My boyfriend and I are playing the game together on PS5. We’ve done everything together from the very beginning, always playing at the same time and never separately, yet we got split up in the main campaign. I’m the host, so I understand that progress is saved to my account, but we just finished Act 3 together, and he didn’t receive any rewards for it. The game kept showing that he had already completed the quest, even though he hadn’t.

This is very frustrating. We tried restarting the entire campaign, but it turns out we can only do so with a new battle.net account, which we don’t want to create. Is there any way to reset progress for both of us, or is there a solution to this issue?

We’ve purchased both the base game and the expansion, and this is really disappointing for us